Alumina-transition metal oxide ceramic products of enhanced ductility

ABSTRACT

High-density, finely grained aluminous ceramic compositions are produced by maintaining mixtures of finely divided alumina precursors and transition metal oxide precursors under pressure at a temperature high enough to decompose the precursors to ceramic oxides but below the melting point of the resulting ceramic oxide composition. The novel ceramic compositions produced by this process are characterized by enhanced ductility at temperatures below their melting points, and are therefore useful in the production of ceramic objects by deformation processes of hot forming and the like. They can be heated to temperatures nearer their melting points for crystal growth and creep-resistance.

United States Patent 1 3,776,744 Clendenen [4 Dec. 4, 1973 [54] ALUMINA-TRANSITION METAL OXIDE 3,442,994 5/1969 Herbert 264/66 MI PRODUCTS 01: ENHANCED 3,676,079 7/1972 Morgan 264/332 DUCTIUTY FOREIGN PATENTS OR APPLICATIONS 5] Inventor: a d Clendenen, nd Ca f- 506,964 6/1939 Great Britain 106/73.4 [73] Assignee: Shell Oil Company, Houston, Tex.

Primary ExaminerHelen M. McCarthy {22] Flled June AttorneyHoward W. Haworth et a1. [21] Appl. No.: 263,014

Related U.S. Application Data [57] ABSTRACT Contimmi("Pin-Palt 9 pt. 21, High-density, finely grained aluminous ceramic com- 1970' abandoned whch a commuanomn'pan of positions are produced by maintaining mixtures of 735541 June 1968 abandoned finely divided alumina precursors and transition metal 7 oxide precursors under pressure at a temperature high [52] U.S. Cl 1(2k65/t72;t,216046/6665,216(16g6g enough to decompose the precursors to ceramic 51 l t Cl C044) C 60 ides but below the melting point of the resulting ced s 3 4 ramic oxide composition. The novel ceramic composi- 1 c 2 tions produced by this process are characterized by l l enhanced ductility at temperatures below their melting points, and are therefore useful in the production [56] References Cited of ceramic objects by deformation processes of hot UNITED STATES PATENTS forming and the like. They can be heated to tempera- 2,369,709 2/1945 Baurnann etal 106/66 X tures nearer their melting points for crystal growth 3,379,523 4/1968 Das Chaklader 264/332 and creep-resistance. 3,549,400 12/1970 Lachman 106/65 3,702,881 11/1972 Das Chaklader 264/332 3 Claims, N0 Drawings 1 ALUMlNA-TRANSITION METAL OXIDE CERAMIC PRODUCTS OF ENHANCED DUCTILITY This application is a continuation-in-part of copending application Ser. No. 74,181, filed Sept. 21, 1970,

which in turn is a continuation-in-part of Ser. No. 735,541, filed June 10, 1968 and both now abandoned.

BACKGROUND OF THE INVENTION This invention relates to aluminous ceramic compositions. More particularly, it relates to dense, finelygrained, plastic, multi-component ceramic compositions and a decompositive hot-pressing process for their manufacture, It also relates to processes of deforming the ceramic compositions under relatively mild conditions oftemperature and pressure.

A variety of processes are used for forming ceramic objects. Typical processes are, for example, slip-casting followed by firing, ceramic oxide powder pressing followed by sintering, and hot pressing of ceramic oxide powders. The process chosen for the production of a ceramic object depends to a large extent on the properties desired of the ceramic product. Slip-casting/firing is often employed in the manufacture of ceramic kitchen articles'andbathroom fixtures, where low fabrication costs more than offset the lack of dimensional precision and the porous, low density, low brittlestrength product inherent in the I process. When a dense, harder, dimensionally accurate ceramic product is desired, pressing and hot pressing are often employed. Thus, pressing or hotpressing of a body of a powdered ceramic oxide ora body of porous ceramic oxide contemplates changing the shape of the body by reducing its density, that is, closing up or reducing the pores by a process of compaction- Conventionally, pressing can involve forming a powder of ceramic oxide into the desired shape with a press'and die and then sinte ring theshape'd article. With hot pressing, pressure is applied duringthe'sintering. Conventional hot pressing processes require temperatures at or very near the melting point of the ceramic material, typically 1 ,4 C or higher. These temperaturesseverely limit-the process utility, since they require the. use of diesor working surfacesof graphite, zirconia, or other specialty materials and precludethe 'use'of metal surfaces. Also, products of conventional hot pressing processes can only be further shaped, if desired, at these extremelyhigh temperatures.

A modified hot pressing process which avoids the conventional high temperatures is disclosed in U.S. Pat.

No. 3,379,523 issued Apr. 23, 1968 to Chaklader. In thisprocess, decomposable starting materials are decomposed under pressure at temperatures from 300C to l,000C to give ceramic products such as zirconia. This process generally employs single decomposable substances or. mixtures of one decomposable substance and a metal as starting materials. This process has the disadvantage, when used to prepare aluminous products from aluminum hydroxide, for example, that at the temperatures specified a relatively low density product is produced. The products of this process also, as with conventional hot pressed products, canonlybe further shaped at temperatures very near their melting point, i.e., in the case of alumina, l,800-2,000C. These temperatures are from about 0.90 to about 0.98 of the absolute melting temperature of the ceramic product.

STATEMENT OF THE INVENTION It has now been found that when certain intimate uniform mixtures of finely divided, decomposable aluminum compounds and decomposable compounds of transition metals, especially iron, chromium, and/or titanium, are maintained under pressure of from 3,000 to 10,000 psig and at a temperature within a critical range substantially below the melting point, novel aluminous ceramic compositions of improved physical propertiesare produced. The resulting ceramic compositions are characterized by a high density and a fine grain size. They are capable of being deformed, by virtue of their properties ofhigh plasticity and ductility, at temperatures substantially below their melting point.

DESCRIPTION OF PREFERRED EMBODIMENTS The ductile ceramic compositions produced by the process of this invention comprise mixtures of at least 50% by weight of aluminum oxide, i.e., alumina, and at least 5% by weight of oxide of at least one transition metal. Iron, chromium and titanium are preferred transition metals. The compositions, in all cases, will have a substantial proportion of aluminum oxide, e.g., from about 50% by weight to 95% by weight, preferably from about 50% by weight to about 85% by weight, based on the total ceramic composition. In preferred two-component systems, iron oxide, chromium oxide, or titanium oxide will be present in conjunction with the alumina in an amount of from about 50% by weight to about 5% by weight, most preferably vfrom about 50% by weight to about 15% by weight, based on the total ceramiccomposition. Illustrative two-component mixed oxide ceramic compositions are 50% alumina- 50% ferric oxide, alumina-25% chromic oxide, alumina-20% titanium dioxide, 95% alumina-5% titanium dioxide, and alumina-15% ferric oxide.

In some instances, advantageous properties are imparted to the ceramic composition by incorporating therein a minor proportion of an additional metal oxideof the group iron oxide, chromium oxide, and titanium oxide, thus making a three-component mixture. Illustrative three-component mixed oxide ceramic compositions are 50% alumina-45% ferric oxide-5% titanium dioxide, 80% alumina-l5% ferric oxide-5% titanium dioxide, 70% alumina-15% chromic oxide-15% iron oxide, and alumina-8% ferric oxide-2% titanium dioxide. In general, ceramic compositions consisting essentially of alumina and iron oxide or of alumina, iron oxide and titanium dioxide are preferred. Ceramic compositions consisting essentially from 0 to 5% by weight of titanium dioxide, 15 to 50% by weight of ferric oxide and 50 to 85% by weight of alumina based on the total ceramic composition are especially useful and are particularly preferred.

The mixed metal oxide ceramic compositions of the invention are produced from mixtures of compounds of the metals other than their oxides but which compounds decompose to be oxides at temperatures below the temperature at which the ceramic compositions are produced. Thus, the ceramic oxide compositions are prepared by heating under pressure intimate, uniform mixtures of finely-grained-solid particles of decomposable compounds of aluminum and at least one decomposable compound of transition metals of the group iron, chromium and titanium, to a temperature at which these compounds decompose to give oxides.

Suitable decomposable compounds of aluminum, iron, chromium and titanium are the hydroxides, carbonates and oxalates. The exact proportions of the mixtures of decomposable substances are selected to provide the twoand three-component aluminous ceramic oxide products herein before described following decomposition. For example, to prepare a ceramic product containing 85% by weight of alumina and 15% by weight of iron oxide, any of the following typical mixtures of decomposable compounds of aluminum and iron might be used.

AI2(CO3)3 FCCOgHgo As another example, to produce ceramic products containing 5% by weight of titanium dioxide, by weight of iron oxide and 80% by weight of aluminum oxide, mixtures of decomposable compounds such as the following are very suitable:

The decomposable metal compounds are suitably prepared separately and then intimately mixed or are initially produced in an intimate mixture as by coprecipitating a mixture of metal hydroxides or carbonates from an aqueous solution.

Regardless of the method by which the intimate mixtures of metal oxide precursors are obtained, it is important that the materials be of a fine-grain structure. The average grain size of the ceramic compositions and therefore of the metal oxide precursors, should be no more than about 3 microns in diameter and preferably no more than about 1 micron in diameter. Particles of the proper grain size are frequently produced by the above coprecipitation technique and no further treatment is necessary. In the instances where the grain size is too large, conventional methods of attrition such as ball-milling are suitable for reduction of grain size.

The mixture of finely grained particles of metal oxide precursors is subjected to elevated pressure and a controlled elevated temperature which serve to effect the conversion of oxide precursor to oxide and to form the ceramic oxide compositions. Suitable pressures vary from about 3000 psig to about 10,000 psig with the pressure range from about 4,000 psig to about 8,000 psig being preferred. The optimum temperature to be employed will depend upon the physical properties of the ceramic composition precursor, particularly upon the decomposition point. Although the absolute value of the optimum temperature will vary widely with the nature of the mixed oxides, it has been found that best TABLE I SUITABLE CERAMIC FORMING AND PROCESSING TEMPERATURES Ceramic Composition l5% Fe,0

85% AI O 2% TiO,

Suitable Temperatures I ,000-l 150C l,000- l ,5 00C The resulting compositions are dense, compact, solid bodies characterized by a high density which approaches the theoretical maximum, e.g., at least about of the theoretical maximum density, more often at least about and on occasion over 99% of the theoretical maximum density. Moreover, at the conditions of elevated pressure and temperature, such as those at which they are formed, the ceramic compositions are characterized by a high degree of ductility and plasticity, being deformable without fracture at strain rates of at least 10% per minute and frequently of up to 100% per minute under stress loads, i.e., deforming pressures, of from about 3,000 psig to about 10,000 psig, with total strains of up to 70%. The terms deformation or deforming contemplate substantially changing the shape of a ceramic body in more than one dimension without necessarily substantially changing the density of the body. Thus, the ceramic compositions are deformed by hot-rolling through flat or shaped rollers into flat strips, bars, rods or other articles of desired shape and dimension at temperatures sufficiently low to enable use of metal dies or working surfaces. Conventional techniques of extrusion are also applied to deform these ceramic compositions to fabricate tubes and other articles of more complex cross-sectional areas. Because of the high density of the ceramics, little if any observable contraction takes place on return of the ceramic materials to ambient temperature, Thus, ceramic articles are heat-formed from the present ceramic compositions with dimensions quite close to those ultiresults are obtained when a temperature is employed mately desired and yet the compositions substantially retain those dimensions upon cooling. As a result, the ceramic compositions of the invention are particularly useful in the production, by deforming techniques of forging and stamping and deep drawing of shaped ceramic articles, e.g., furnaces, crucibles and the like, where dimensional control is required and properties of strength and resistance to chemical attack are important. Subsequent to forming the ceramic products into the desired shape, the ductility can be removed by maintaining the products at higher temperatures, e.g., 80% to 90% of the melting temperature, to grow larger grain and impart high-temperature creep-resistance to the products.

To further illustrate the novel compositions of the invention, their production and deformation, the following examples are provided. It should be understood that the details thereof are not to be regarded as limitations EXAMPLE 1 A series of alumina-transition metal oxide ceramic compositions was produced by coprecipitating aluminum and transition metal hydroxides with aqueous ammonia from aqueous solutions of the metal nitrates of calculated concentration. Each precipitate was filtered and dried at 50C to remove free water and placed in a piston-cylinder graphite die. The die was pressurized and heated at a controlled heating rate to calcine and sinter the ceramic. The ceramic products thus formed had a very fine grain structure, of the order of one micron. The compositions of ceramic products thus produced are listed in Table 11, together with the pressure and final temperature of their production and the heating rate employed. The term T/T measures the ratio of the temperature employed to the melting temperature and the term dfld measures the ratio of the actual density of the ceramic to the theoretical density.

TABLE 11 Heating Composition Pressure Temp. Rate T/ d,/ Product wt psig "C CIH Tm] "'theor A 50% 10,0, 4,000 1,100 500 0.70 0.99

50% Fe o B 50% A1,O 8,000 1,100 300 0.70 0.99

50% Fe Q, C 50% A1 0 4,000 1,050 400 0.68 0.99

50% R 0 D 90% A1 0 4,000 900 400 0.55 0.85

TiO, E 85% A1 0 4,000 1,150400 0.63 -0.95

Fe O F 50% A1,O 4,000 1,300 400 0.65 0.99 I

50% Cr O, G 75% A1 0; 4,000 1,200 400 0.74 0.99

25% F12 0 H 65% A1 0 4,000 1,201) 400 0.74 0.99

35% Fc,O,-, 1 95% A1 0 4,000 1,300 400 0.70 0.95

5% F0 0 84% A1 0 .1 14% Fe,O 4,000 1,100 400 0.60 0.96

2% T10, K 85% A1 0,, 3,000 950- 400 0.55 0.85

15% Fc,0 I

74% AliO i L 24% mo, 4,000 1,150 400 0.64 0.99

2% TiO, 50% Al,0a M 30%- Fe,0 4,000 1,400 400 0.75 0.99

(r 0 64% A1 0 I I I N 34% mo. 4,000 1,100 400 0.62 0.99

EXAMPLE 11 The ductility of ceramic products produced by the procedure of Example 1' was determined using specimensof one-fourth inch by one-fourth inch by one-half inch dimensions cut from samples previously produced. The specimens were placed in a graphite element furnace and heated from ambient atmospheric temperature (30C) to a temperature from 0.5 to 0.7 ofthe melting temperature, i.e., to a T/T value of from 0.5 to 0.7, and pressure was applied to one end of the specimen to produce a constant strain rate. The products evaluated, the temperature of evaluation, the maximum strain rate without fracture, the pressure applied to maintain that strain rate, and the highest total strain without fracture are provided in Table 111.

TABLE III Pressure Applied at Strain Total Temp. Strain Rate Strain Product "C 'lfl' Rate %/m|n psl A 1,100 0.71 3,000 100 70 B 1,000 0.66 8,000 70 70 C 900 0.61 18,000 20 40 D 1,200 0.70 8,000 20 22 F 1,300 0.65 30,000 20 25 G 1,200 0.74 10,000 20 H 1,200 0.74 10,000 10 1 1,300 0.70 10,000 20 25 M 1,400 0.75 6,000 20 50 In contrast to the large low temperature plastic deformations observed with products according to this invention, l-leuer et a1, 52, .1. Am. Ceramic Soc., 468 (1969) show that aluminas prepared by conventional hot-pressing methods require temperatures of from 0.87 to 0.95 of absolute melting (i.e., 1,750C 1,920C) to give strain rates of from 7 to 36%lmin.

EXAMPLE I11 Heating-treating to remove ductility A sample of product A, Table I1, (50% A1 O -50% Fe O was annealed at 1,300C (T/T,, 0.81) for 72 hours. Studies of fracture surfaces of this material with an electron microscope indicated that prior to annealing this material had a grain size of the order of 0.1 microns whereas after annealing the grain size had grown to some 50 to 100 microns in diameter. When tested under the same conditions as product A, Table 111, line 2 (1,000C at a strain rate of 70%/min) this material evidenced no plastic deformation in contrast to a 70% plastic deformation observed prior to annealing.

EXAMPLE [V A body of pure alumina was prepared in accord with the general procedure of Example 1. A solution of Al(NO was treated with NH.,OH to precipitate A1(OH) which after washing and drying was placed in a piston-cylinder die and heated at 1,300C and pressed at 4,000 psi to decompose the hydroxide and give a 95-l dense solid body of alumina. The ductility of this body of alumina was measured by the method of Example ll of the above-noted patent application with the following result: at 1,200C (T/T,, 0.62), when a strain rate of 5%lminute was applied, no measurable deformation was noted up until a pressure of 12,000 psi was applied at which point the sample fractured.

EXAMPLE V A sample of pure ferric oxide (Fe O was prepared in accord with the general procedure of Example 1 by precipitating Fe(O1-l) from Fe(NO with Nl-hOl-l and then heating and pressing the Fe(OH) at 1,1 10C and 4,000 psi to give a 99+% dense Fe O body. This body was tested by the method of Example 11 and found to exhibit no notable deformation below its melting point and fractured at 1,050C when a pressure of 60,000 psi was reached. The strain rate employed was 5%.

EXAMPLE VI A body of alumina/10% zirconia was prepared in accord with the procedure of Example 1 by precipitating a mixture of A1(O1-l) and Zr(OH) and Zr(OH).,

from a solution of the nitrates using NH OH and heating and pressing the mixed hydroxides at 1,400C and 4,000 psi to give a 95+% dense body of 90% Al O /l% ZrO,. This body was tested by the method of Example 11 and found to have no ductility at temperatures below its melting point, fracturing when a pressure of 32,000 psi was gradually applied at 1,300C (T/T 0.67).

EXAMPLE Vll A body of pure alumina was prepared with general procedure of Example l by treating a solution of Al(NO,-,) with Nl-LOH to precipitate Al(OH) which after washing and drying was placed in a pistoncylinder die and heated at 1,300C and pressed at 4,000 psi to decompose the hydroxide and give a 95+% dense solid body of alumina. The ductility of this body of alumina was measured by placing the body between the platens of a press, heating the body at C/minute while applying a constant load of 4,000 psi. Strain rates were measured at hundred degree intervals and the first recordable deformation occurred at 1,600C, (T/T 0.81 at a strain rate of 0.07%/min. At 1,700C, (T/T,,,) 0.85, a strain rate of l%lminute was recorded, and the sample fractured at l ,7 25C, terminating the experiment.

EXAMPLE Vlll A sample of high density commercial alumina ceramic (General Electric Companys LUCALOX Ceramic) was placed under a constant load of 10,000 psi and heated to 1,800C at a heating rate of l0C/minute. The sample showed no noticeable deformation below 1,700C and at 1,800C, (T/T 0.9, the deformation rate was less than l%/minute.

EXAMPLE [X A sample of pure Fe O was prepared in accord with 8 Example V was tested. A constant load of 4,000 psi was applied and the heating rate was maintained at 10C per minute. At 1,060C, (T/T 0.72, a maximum strain rate of 0.6%lminute was observed and at 1,125C, (T/T 0.75, the sample failed.

EXAMPLE X A mixture of aluminum oxide and chromium oxide outside the range of mixtures claimed in this application, 4.8% Cr O 95% A1 0 was prepared by the process of this invention and tested at a 4,000 psi constant load and a 10C/minute heating rate. At (T/T 0.72 a deformation of O.1%/minute was noted and at (T/T,,,) 0.78 a strain rate of 1% per minute was noted.

1 claim as my invention:

1. A ceramic product consisting essentially of from 50% by weight to by weight of aluminum oxide and from 15% by weight to 50% by weight of at least one oxide selected from the group consisting of ferric oxide, chromium oxide and titanium oxide, said composition having an average grain size of not more than 3 microns in diameter, a density of at least of the theoretical maximum and further characterized as being deformable without fracture at strain rates of at least 10% per minute at temperatures in the range of from 55% to 75% of its melting temperature.

2. The ceramic product of claim 1 wherein the oxide selected from the group consisting of ferric oxide, chromium oxide and titanium oxide is ferric oxide.

3. The ceramic product of claim 1 wherein the oxide selected from the group consisting of ferric oxide, chromium oxide and titanium oxide is chromium oxide. 

2. The ceramic product of claim 1 wherein the oxide selected from the group consisting of ferric oxide, chromium oxide and titanium oxide is ferric oxide.
 3. The ceramic product of claim 1 wherein the oxide selected from the group consisting of ferric oxide, chromium oxide and titanium oxide is chromium oxide. 